Consider adding AppendFormattable to StringBuilder

[paulomorgado]

I see a lot of code like this:

new StringBuilder().Append$"Now = {DateTime.Now}");

(Well, not exactly like this, but you get my point)

The convenience of string interpolation leads to an unnecessary string instantiation.

Adding an AppendFormattable method to StringBuilder (even if just as extension methods) would keep the benefits of string interpolation but without the drawbacks of the intermediate invocations of string.Format.

public static class StringBuilderExtensions
{
    public static StringBuilder AppendFormattable(this StringBuilder builder, FormattableString formattableString)
        => builder.AppendFormattable(null, formattableString);

    public static StringBuilder AppendFormattable(this StringBuilder builder, IFormatProvider formatProvider, FormattableString formattableString)
        => builder.AppendFormat(formatProvider, formattableString.Format, formattableString.GetArguments());
}

Now I can do this:

new StringBuilder().AppendFormattable($"Now = {DateTime.Now}");

[stephentoub]

The convenience of string interpolation leads to an unnecessary string instantiation.

In your example, even with the suggested AppendFormattable there's still an object[] allocation and a boxing allocation for the DateTime, plus the additional FormattableString allocation. So it's actually the same number of allocations, albeit potentially for slightly less total memory.

The more efficient way to write this would be:

sb.Append("Now = ").Append(DateTime.Now);

As there's currently no DateTime-based overload of Append, this will still allocate the boxing object, but it won't incur the allocation for the object[] nor for the FormattableString, and it won't incur the formatting overhead associated with parsing the format string and the like. We could choose to optimize Append(object) internally as well to be more efficient for something like DateTime.

I'd prefer to see this addressed with something like #28945 and dotnet/csharplang#2302.

[paulomorgado]

I'm not trying to fix the boxing of value types.

I'm just trying to provide something that works today based on the intuition that, in the general case, the cost of the instantiation of a System.Runtime.CompilerServices.FormattableStringFactory+ConcreteFormattableString will be less than the cost of a throw away string.

And the new method name is because the current compiler prefers string to IFormattable as the target type of the conversion of the interpolation.

But, of course, those proposals, when implemented are better.

[stephentoub]

based on the intuition that

You can just measure :)

private StringBuilder _sb = new StringBuilder();
private DateTime _dt = DateTime.Now;

[Benchmark]
public void AppendFormat()
{
    _sb.Clear();
    FormattableString fs = $"Now = {_dt}";
    _sb.AppendFormat(fs.Format, fs.GetArguments());
}

[Benchmark]
public void AppendString()
{
    _sb.Clear();
    string s = $"Now = {_dt}";
    _sb.Append(s);
}

[Benchmark]
public void AppendAppend()
{
    _sb.Clear();
    _sb.Append("Now = ").Append(_dt);
}

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
AppendFormat	316.0 ns	1.41 ns	1.25 ns	0.0138	-	-	88 B
AppendString	305.4 ns	0.72 ns	0.64 ns	0.0162	-	-	104 B
AppendAppend	258.2 ns	3.09 ns	2.74 ns	0.0138	-	-	88 B

[paulomorgado]

I can even measure with something more close to the code I was reviewing. 😄

private readonly StringBuilder sb = new StringBuilder();
private readonly int age = 52;
private readonly float heigh = 1.73f;
private readonly string firstName = "Paulo";
private readonly string lastName = "Morgado";

[Benchmark]
public void AppendFormat()
{
    this.sb.Clear();
    FormattableString fs = $"My name is {this.firstName} {this.lastName}. I'm {this.age} years old and {this.heigh}m tall.";
    this.sb.AppendFormat(fs.Format, fs.GetArguments());
}

[Benchmark]
public void AppendString()
{
    this.sb.Clear();
    string s = $"My name is {this.firstName} {this.lastName}. I'm {this.age} years old and {this.heigh}m tall.";
    this.sb.Append(s);
}

[Benchmark]
public void AppendAppend()
{
    this.sb.Clear();
    this.sb
        .Append("My name is ")
        .Append(this.firstName)
        .Append(" ")
        .Append(this.lastName)
        .Append(". I'm ")
        .Append(this.age)
        .Append(" years old and ")
        .Append(this.heigh)
        .Append("m tall.");
}

Method	Mean	Error	StdDev	Median	Gen 0	Gen 1	Gen 2	Allocated
AppendFormat	1,764.8 ns	101.53 ns	299.36 ns	1,666.4 ns	0.0496	-	-	209 B
AppendString	2,077.2 ns	138.18 ns	398.68 ns	2,026.4 ns	0.0763	-	-	321 B
AppendAppend	987.5 ns	29.11 ns	84.44 ns	976.4 ns	0.0172	-	-	72 B

By the way, AppendAppend was my first recommendation.

[stephentoub]

The StringBuilder.Append{Line} overloads for string interpolation added in .NET 6 address this. Given that, I'm going to close this issue. Thanks.